1. Field of the Invention
The present invention is related to data communication in spread spectrum systems and applications.
2. Related Art
Spread spectrum techniques have become increasingly important in both commercial and military communication, navigation, and test systems. In a spread spectrum system, a transmitted signal is "spread" over a wider frequency band than the minimum bandwidth required to transmit the data being sent. For example, a carrier (or baseband) signal with a bandwidth of a few kilohertz is distributed over a wider bandwidth of several megahertz.
In direct sequence spread spectrum systems, data symbols are conveyed by directly modulating the digital code sequence of a wideband carrier waveform signal. These digital codes are usually phase modulation sequences that have a chip rate much higher than the information bandwidth of the data signal.
Different types of modulation are used to convey data symbols over a spread spectrum channel in direct sequence spread spectrum systems. Two common types of modulation are phase-shift-keying and frequency-shift keying. In phase-shift keying (PSK) the phase of a carrier signal in the time-domain is modulated to represent a data symbol. Different types of phase-shift keying include binary phase-shift keying (BPSK), quadriphase-shift keying (QPSK), and multi-level M-ary phase-shift keying. Frequency-shift keying (FSK) modulates a carrier waveform signal by different frequencies representative of respective data symbols. For example, in binary FSK, the choice of one of two frequencies is used to represent either a zero or a one. See, e.g., Robert C. Dixon, Spread Spectrum Systems with Commercial Applications, 3rd. Ed. (John Wiley & Sons, Inc.: 1994), incorporated in its entirety herein by reference.
Wireless communication networks have adopted spread spectrum techniques. For example, satellite networks support multiple spread spectrum channels linking multiple terminals. Conventional direct sequence modulation techniques, however, can require expensive analog hardware driving up the cost of terminal equipment. Receiver costs are increased further as direct code sequences become more complicated to protect against interference, to provide privacy, and to reduce noise. Hub networks which require a central control station are sometimes required to coordinate communication.
What is needed is a new form of data modulation for conveying data over a spread spectrum channel.